The capture of particulate matter or aerosols in air or other fluid streams is of concern in two entirely different contex: first, as an analytical tool to determine the type and concentration of such particulate/aerosol material, and secondly as a method of cleansing the fluid stream for subsequent use. Additionally, there are occasions in either case where classification of particulate matter by size is desired. For example, the detection of airborne biological or chemical warfare agents, the detection of biological contamination in confined spaces such as aircraft or hospitals or the detection of industrial pollutants (either in ambient air or in stacks) may be required in various and different scenarios.
Much effort has been expended in the past in the detection and classification of particulate matter or aerosols, generally in air or other gaseous streams. Impactors have been used for collecting aerosol particles for many decades. In the earliest embodiments, a stream of gas containing the particles was accelerated towards an impactor plate. Due to their inertia, the particles hit the receptor plate and were collected there while the gas deflected to the side. Therefore, only heavy particles were collected while particles below a "cut-off threshold" size were carried away by the air stream. This concept has been refined over the years, as in U.S. Pat. No. 3,901,798, wherein suspended particles in an air stream are caused to flow along a path having one or more sharp turns. Smaller particles with less mass and inertia negotiate the turn and continue along the deflected path for detection. Larger particles with greater mass and inertia fail to negotiate the turn and continue along the original direction for separation and/or detection.
However, a significant cause of inaccuracy in such impactors results from the deposition of particles on surfaces of the impactor other than the intended collection surfaces. This phenomenon reduces the slope of the particle collection efficiency curves for the impactor stages and reduces the accuracy of measurement of total particle mass concentration and of the size-fractionation of particles since such losses cannot be accurately estimated for aerosols having varying size, shape or chemistry. Additionally, particles may either reentrain in the airstream or bounce from the collection surface upon impact.
To remedy this problem "virtual" impactors have been developed that separate particulate matter in a fluid stream by forces other than impaction, and hence reduce the inaccuracies induced by particle deposition on the device. These devices may operate on a number of different principles, but all avoid actual "impact" and all virtual impactors rely on differences in mass to induce inertial separation. In such devices, the particulate-laden fluid stream (generally air or other gas) is directed toward a surface presenting an obstruction to the forward movement of the fluid stream. In a laminar flow device, the particles are carried along in the air stream with the air molecules, but when the air changes direction (to avoid the obstruction), the heavier particulate matter will be separated from the main portion of the air stream. A region of relatively stagnant air (a "dead air zone") is created in the air stream, comprising a minority of the air stream, but which contains a majority of the particulate matter. The "virtual impact surface" is formed when the air stream separates into major and minor flow streams, such that fine particles remain entrained in the deflected major portion of the air stream, while more dense particles with greater inertia remain in the stagnant air zone in the minor portion of the air stream, where they can be captured.
Because many of the prior art virtual impactors did not actually collect particles themselves, but merely redirected them into two different air streams according to their mass, they were essentially free of the problems of particle bounce and reentrainment associated with actual impact devices.
Virtual impactors have been devised in a number of embodiments. For example, U.S. Pat. No. 4,670,135 discloses a plurality of virtual impactors each having an inlet nozzle and a receiving tube arranged on a housing or frame for parallel flow and which are utilized to obtain a high flow rate while keeping the flow rate through each individual virtual impactor at a reasonable value. Similarly, U.S. Pat. No. 5,425,802 discloses a s virtual impactor with a nozzle for accelerating an entering airstream, particle receiving means positioned downstream from the nozzle means, and a chamber in fluid communication with the gap between the nozzle means and the receiving means.
In an effort to enrich gases with aerosol particles to certain orders of magnitude for purposes of classification, the virtual impactor of U.S. Pat. No. 5,533,406 comprises a generator plate with an inlet side and outlet side, with a receptor plate disposed parallel with the generator plate so as to define a space therebetween, with each having at least one nozzle in the form of a slit which is curved to form a closed loop. Classification of aerosol particles is also provided by U.S. Pat. No. 3,901,798, wherein a first plate defines a first orifice, and a second plate spaced from and downstream from the first plate defines a second orifice larger than and substantially concentrically disposed with respect to the first orifice. Tubular means in the plane of the second orifice defines a third orifice and an annular passage between the second orifice and tubular means.
A dichotomous virtual impactor is disclosed in U.S. Pat. No. 4,767,524, wherein an impaction plate is replaced by a region of stagnant air in a receiving probe. A virtual surface is formed by deflected streamlines that are similar to those in conventional solid plate impactors. The fine particles follow the streamlines of the major air flow, while the coarse particles pass into the forward minor flow region.
As is apparent from a review of the prior art, the technology of virtual impactors is well known, and advances in the art are occasioned by new designs creating more efficient or more specific separations. The apparatus of the present invention accomplishes both.